High density printed circuit boards are generally constructed with several electrically conductive layers separated by dielectric layers. Some of the conductive layers are used to supply power and ground voltages. The remaining conductive layers are patterned for electrical signal interconnections among integrated circuit chips. Layer-to-layer interconnections are achieved by means of through-holes plated with electrically conductive material. In high density printed circuit boards it has been normal practice to provide interconnections between adjacent conducting layers, which interconnections are commonly known as "vias".
In U.S. Pat. No. 3,740,678; entitled: Strip Transmission Line Structures; issued: June 19, 1963, a triplate circuit board construction is shown in which X-Y signal planes form a repeating basic core structure in a multi-laminated, high density circuit board.
Such a circuit board construction has been useful in transmitting high frequency signals, and relies upon alternating dielectric mediums to achieve its many benefits.
One of the dielectric sheets comprises an epoxy glass having a relatively high dielectric constant.
Such a material is useful in providing structural stability to the other dielectric sheet of the core which comprises FEP Teflon material or polyethylene, which provide more desirable electrical characteristics.
The aforementioned triplate construction has several drawbacks: (1) it is not a true triplate configuration in which X-Y signal planes are disposed about a conductive power or reference plane (although true triplate constructions have been achieved by other fabrication techniques, they have not been successful in providing such constructions with low dielectric materials); (2) there is no teaching in the above-mentioned patent how vias can be fabricated between individual signal planes embedded as a core within the multi-laminated board structure; (3) it would be more desirable to replace the epoxy-glass resin layers of the patented structure with dielectric materials having lower dielectric constants, without jeopardizing the structural strength of the circuit board assembly; (4) if an electrical short should develop within any one of the cores of the assembly, the whole assembly must be discarded. Such electrical short circuits occur with greater prevalence in high performance circuit boards as layers become thinner and signal line density is increased.